The present disclosure relates to a complex second-order integrator, and more particularly to a complex second-order integrator suitable for a continuous-time ΔΣ modulator, etc.
Oversampling A/D conversion, which is in widespread use in the front-end of communications equipment, conversion of audio signals, etc., is a circuitry technology essential for the current communications, video, and audio signal processing circuits. One type of oversampling A/D converters is a continuous-time delta-sigma (ΔΣ) A/D converter (CTDS-ADC) having a continuous-time filter.
In a general CTDS-ADC, an input signal passes through n-cascaded integrators (continuous-time filters) and the signal is quantized by a quantizer. The digital output of the quantizer is fed back to the n integrators after being converted to an analog current signal by n D/A converters. In the CTDS-ADC, having no switch in its analog circuit portion, the voltage can be reduced. Also, it is unnecessary to place a prefilter that is normally necessary when a sampling filter is used. Having these features, the CTDS-ADC is suitable for applications to communications systems, and thus recently application development and research have been actively conducted.
In communications equipment, etc., a complex filter is often used for removal of an image signal. A typical complex filter has a configuration where integrators respectively receiving I signal and Q signal displaced 90° in phase from each other are coupled together with a coupling circuit (see U.S. Pat. No. 4,914,408, for example). While a coupling circuit is generally formed of active elements such as transistors, a coupling circuit that couples first-order integrators together can be formed of passive elements, specifically resistance elements, without use of active elements (see Jan Crols and Michiel Steyaert, “An Analog Integrated Polyphase Filter for a High Performance Low-IF Receiver,” Digest of Technical Papers, Symposium on VLSI Circuit, pp. 87-88, 1995, for example).
The inventor of the present disclosure has found the following problems on the conventional CTDS-ADC. In order to improve the resolution and SN performance of the CTDS-ADC, the filter order for removal of quantization noise must be increased, and this necessitates operational amplifiers of the number corresponding to the increased filter order. Moreover, when it is intended to implement a CTDS-ADC provided with a complex coefficient, the number of operational amplifiers must be doubled. As described above, a coupling circuit that couples first-order integrators together can be formed without use of any operational amplifier. However, it is unknown whether a coupling circuit that couples high-order integrators each including one operational amplifier can be formed without use of any active element.
In other words, to improve further the performance of the CTDS-ADC by putting the CTDS-ADC in a complex form, a number of operational amplifiers must be used. However, increase in the number of operational amplifiers will increase the circuit scale and the power consumption, causing a bottleneck in improving the performance of system LSIs applied to mobile communications equipment, etc.